Ink jet printers generally function in one of two modes: continuous stream or drop-on-demand. Ultrasonic printheads have been described in detail in a number of commonly-owned U.S. Patents, including Pat. Nos. 4,719,476 and 4,719,480, whose contents are herein incorporated by reference.
These patents describe the generation of capillary surface waves on the surface of the ink by various means, such as acoustically, mechanically, thermally, or electrically, to periodically perturb the free surface of a volume of liquid ink at a suitably high excitation frequency f.sub.c. If the amplitude of this oscillating pressure equals or exceeds a critical "onset" amplitude level, one or more standing capillary waves are generated on the free surface of the liquid ink. Capillary waves, as defined therein, are waves which travel on the surface of a liquid in a region where the surface tension of the liquid is such a dominating factor that gravitation forces have negligible effect on the wave behavior. The patents further discuss the production of the waves by parametric excitation of the liquid, so that their frequency f.sub.sc is equal to one half of the excitation frequency (f.sub.sc =f.sub.c /2). The capillary surface waves are periodic and generally sinusoidal at lower amplitudes, and they retain their periodicity but become non-sinusoidal as their amplitude is increased.
The systems of these patents provide acoustic transducers immersed in the liquid for generating a standing capillary wave at the surface of the ink, and addressing mechanisms for selecting the sites from which droplets are to be ejected, to locally alter the surface properties of selected crests at those sites. For example, the local surface pressure acting on the selected crests or the local surface tension of the liquid within the selected crests may be changed in order to cause droplets to be ejected in a controlled manner from the selected crests.
Acoustic ink printers are also disclosed in commonly-owned United States patent No. 4,748,461, the contents of which are also incorporated herein by reference. This patent discusses the generation of radially directed capillary waves at the surface of the liquid ink, by an electrode structure, to coherently interact with the capillary waves generated by the focussed output of an acoustic generator immersed in the liquid, in order to enable the ejection of ink drops from the pool of liquid ink. In this arrangement, the maximum displacement of the electrodes from the acoustic wave center is limited by the damping of the capillary waves resulting from the viscosity of the liquid.
In an acoustic ink printer, the ejection of droplets from the surface of the liquid ink has also been found to result in the generation of capillary waves that radiate, for example, from the locus on the surface of the liquid from which the droplet was ejected. It has further been found that the repetition rate of the printhead transducers is limited by the necessity that these capillary waves must die out before a new droplet may be ejected.